The present invention relates to driver circuits. More particularly, the present invention relates to driver circuits with an inductive load.
Current driver circuits and other driver circuits are generally known and used for many applications. A basic current driver circuit 100 of the prior art is shown in FIG. 1. The driver circuit 100 has a signal input 105. The signal input 105 is connected to the positive terminal of amplifier 115 and to one end of resistor R1110. The other end of resistor 110 is connected to ground. The output of amplifier 115 is connected to one end of driver resistor 120. The other end of resistor 120 is connected to the base of transistor 125. The negative terminal of amplifier 115 is connected to the emitter of amplifier 125 and to one end of current sensor resistor 140. The other end of resistor 140 is connected to ground. A voltage source 130 is connected to one end of an inductance load 135. The other end of the inductance load 135 is connected to the collector of transistor 125.
In operation, transistor 125 of the current driver sinks a current through the inductive load 135. Signal input 105 applies input signal vi to the positive terminal of amplifier 115 and to resistor 110. The voltage at the negative terminal of amplifier 115 is approximately equal to the voltage applied to the positive terminal of the amplifier. The voltage at the emitter of the transistor is applied to the negative terminal of the amplifier, which in turn sets the voltage at the base of the transistor. In one embodiment, the voltage at the base of the transistor is approximately 0.7 volts more than the emitter. Assuming operation in the active state for transistor 125, when the input signal vi is low, a voltage difference is applied across resistor 120 inducing a base current iB. When the base current iB is induced at the base, a current iC is induced at the collector whereby iB=((1xe2x88x92xcex1)/xcex1)* iC, where xcex1 is a constant for the particular transistor. The collector current iC drives the inductive load 135. When the input signal vi goes high, the voltage difference across resistor 120 increases and induces a larger current iB. This induces a larger iC current to flow through the inductive load 135.
The current response to driver circuit 100 is shown in FIG. 2 and displays several disadvantages of drive circuit 100. As shown in FIG. 2, the driver circuit 100 displays a slow rise time and a slow recovery time. A faster rise time can be achieved by increasing the amplifier supply voltage 130. However, increasing the supply voltage results in decreasing the efficiency of the driver circuit. The current response of FIG. 2 also displays a large negative spike voltage characteristic. Such a negative spike voltage of the driver circuit 100 may damage the drive transistor unless a capacitor or diode is used to eliminate it.
Another type of driver of the prior art is a current push/pull driver. A basic current push/pull driver circuit 300 is shown in FIG. 3. An input signal is connected to resistor 310 and the positive terminal of driving amplifier 315. The output of amplifier 315 is applied to one terminal of resistor 320 and inductive load 330. The negative terminal of the amplifier 315 is connected to the other terminal of resistor 320, inductive load 330, and to one terminal of current sensing resistor 325. The other terminal of current sensing resistor 325 is connected to ground.
In operation, the signal input 305 applies signal vi to resistor 310 and to the positive terminal of driving amplifier 315. The output of the amplifier 315 is applied to one end of resistor 320 and one end of the inductive load 330. The voltage at the negative terminal of driving amplifier 315 is approximately the same as the voltage at the positive terminal of driving amplifier 315. The voltage at the negative terminal of the drive amplifier applies a voltage to one terminal of current sensing resistor 325. The voltage difference across current sensing resistor 325 induces a current through resistor 325 towards the grounded terminal of the resistor and through resistor 320 towards resistor 325. Resistor values for resistor 320 and 325 are chosen such that the current driven through resistor 325 will be more or less than the current through resistor 320 depending on whether the input signal goes high or low. When input signal vi is low, the voltage difference across resistor 320 induces a current across resistor 320 towards node 340. This provides a current across resistor 320 smaller than the current drawn by current sensor resistor 325. As a result, current is pushed through inductor 330 towards node 340. When input signal vi is high, the voltage difference placed across resistor 320 is now higher then when vi was low and higher than the current drawn by current resistor 325 away from node 340. As a result, current is pulled through inductor 330 away from node 340.
The current response of the push/pull driver circuit 300 is shown in FIG. 4. The current response of circuit 300 is improved over the current response of circuit 200. The negative spike voltage characteristic has been eliminated due to the push/pull characteristic of circuit 300. The push/pull operation to the inductive load operates to remove some of the energy stored in the inductive load. Current driver 300 still possess a slow rise time characteristic is shown in FIG. 4. Though the rise time of circuit 300 could be improved by increasing the supply voltage, this would decrease efficiency and require additional elements such as heat sink components.
What is needed is an improved circuit for driving an inductive load. The circuit should generate a high enough voltage to drive an inductive load at high speeds and display a favorable rise time. Further, a driving circuit is needed that can provide a low level of noise, high frequency capability, and be otherwise configurable to meet different system requirements as needed.
A driving circuit for driving an inductive load in accordance with the present invention includes a high frequency driver and low frequency driver. A low frequency component and high frequency component is taken from an input signal. The separate low and high signal components are input to a low frequency driver and high frequency driver, respectively. The outputs of the high frequency and low frequency drivers are combined by combination circuitry. In one embodiment of the present invention, the high frequency component is also amplified by the combination circuitry. The combined signals generate a high voltage signal that drives an inductive load at fast speeds. The driver circuit of the present invention may be configured to provide low noise at low frequencies, pass band frequency response at the load terminal, different AC and DC open loop gains, and other characteristics depending upon system requirements.